Method of measuring humidity using an electrochemical cell

Abstract

In a method of measuring humidity by using an electrochemical cell, the method comprises applying a voltage across the pair of electrodes to obtain a characteristic curve between the voltage applied and electrical current. The characteristic curve starts with a first flat portion in which a first diffusion limit current remains generally constant within a predetermined voltage range and passes through a point of inflection in which a second differential derivative of the characteristic curve falls on zero at certain voltage, and ends up with a second flat portion in which a second diffusion limit current remains generally constant within a predetermined voltage range. The point of the inflection is uniquely determined regardless of a humidity level of the gas at a constant temperature and oxygen concentration. The humidity level in the gas is obtained by reading the point of the inflection, and the intensity of the first and second diffusion limit current and then comparing the diffusion limit current with the inflection as a reference current.

What is claimed is: 
     
       1. In a method of measuring humidity by using an electrochemical cell which includes an oxygen-ion conductive solid electrolyte, a porous negative electrode and a porous positive electrode tightly placed on a surface of the electrolyte, the negative electrode being arranged to be partly exposed to a humidity laden gas so that an oxygen component in the gas is transferred from the negative electrode to the positive electrode by a pumping action according to a voltage applied across the pair of electrodes; the method comprising:   applying a voltage across the pair of electrodes to obtain a characteristic curve between an intensity of the voltage applied and that of an electrical current, the characteristic curve continuously originating from a first flat portion in which a first diffusion limit current remains generally constant with a predetermined voltage range and always passes through a point of inflection with the increase of the applied voltage in which a second differential derivative of the characteristic curve falls on zero at a certain voltage, and ends up with a second flat portion in which a second diffusion limit current remains generally constant with a predetermined voltage range, the point of the inflection being uniquely determined regardless of a humidity level in the gas at constant temperature and oxygen concentration;   reading the point of the inflection, and at the same time, finding the intensity of the first diffusion limit current and the second diffusion limit current; and   determining the humidity level in the gas from the fact that the humidity corresponds to a differential value either between the first diffusion limit current and the point of inflection or between the second diffusion limit current and the point of inflection.   
     
     
       2. A method in accordance with claim 1 wherein said electrodes are located on the same surface of the solid electrolyte. 
     
     
       3. A method in accordance with claim 1 wherein said solid electrolyte is located on a ceramic substrate heater to facilitate the conductivity of the oxygen ions. 
     
     
       4. A method in accordance with claim 3 wherein said substrate heater comprises fired alumina and tungsten. 
     
     
       5. A method in accordance with claim 1 wherein the solid electrolyte comprises a solid solution or zirconium oxide and yttrium. 
     
     
       6. A method in accordance with claim 1 wherein said negative electrode is at least partially coated with a porous alumina layer and a glazed layer. 
     
     
       7. A method in accordance with claim 1 wherein said negative electrode provides a porous connection portion which serves as an aperture for limiting the oxygen diffusion and aqueous diffusion. 
     
     
       8. A method in accordance with claim 1 wherein said electrodes are covered by a solid electrolyte provided with a hole communicating a positive electrode portion and the gas to be measured.